MCPToolCallResult is rejected as a type expression by strict type checkers #564

Summary

MCPToolCallResult in langchain_mcp_adapters/interceptors.py is defined as
two plain module-level assignments inside if LANGGRAPH_PRESENT: / else:, with
no TypeAlias marker. Strict type checkers (Pyright/Pylance in strict mode,
mypy --strict) reject any use of the name in a type expression:

Variable not allowed in type expression (reportInvalidTypeForm)

This affects anyone implementing the public ToolCallInterceptor protocol,
whose signature (handler: Callable[[MCPToolCallRequest], Awaitable[MCPToolCallResult]]) -> MCPToolCallResult)
cannot currently be annotated without # type: ignore / # pyright: ignore
suppressions on the downstream side.

Reproduction

from collections.abc import Awaitable, Callable

from langchain_mcp_adapters.interceptors import (
    MCPToolCallRequest,
    MCPToolCallResult,
)


async def my_interceptor(
    request: MCPToolCallRequest,
    handler: Callable[[MCPToolCallRequest], Awaitable[MCPToolCallResult]],
) -> MCPToolCallResult:  # <-- Pyright: reportInvalidTypeForm
    return await handler(request)

Running Pyright (default or strict) against the snippet above yields two
reportInvalidTypeForm errors on the MCPToolCallResult annotations.

Root cause

MCPToolCallResult is currently defined as:

if LANGGRAPH_PRESENT:
    from langgraph.types import Command
    MCPToolCallResult = CallToolResult | ToolMessage | Command
else:
    MCPToolCallResult = CallToolResult | ToolMessage

Because LANGGRAPH_PRESENT is a runtime bool populated from a
try/except ImportError, type checkers cannot narrow to a single branch and
see two competing implicit aliases with no TypeAlias marker — so they
refuse to treat the name as a valid type form.

Environment

• langchain-mcp-adapters 0.3.0 (also reproduces on main)
• Pyright / Pylance (default and strict mode)
• Python ≥ 3.10

Proposed fix

Annotate both branches with typing.TypeAlias (available since 3.10, within
the project's declared Requires-Python). Non-breaking — preserves the
runtime symbol and behavior. PR to follow.